In oil well completions, it is common to line the borehole with a tubular metal liner and to cement the annulus between the liner and the borehole by injecting a liquid cement slurry under pressure through the bottom end of the liner into the annulus between the liner and the wellbore. The liquid slurry is moved up the annulus between the liner and the wellbore under pressure and subsequently sets up in the annulus to support the liner in the well bore. There are limitations as to the length or height of a column of cement which can be pumped into a well annulus. Where the length of the liquid cement column in an annulus is too long it is not uncommon to insert a stage cementing collar along the length of the liner. In this instance, a liquid cement slurry is first located between the end of the liner, and the stage collar. Next, the stage collar is opened and liquid cement slurry is injected into the annulus located above the stage collar. After cementing the upper annulus above the stage collar, the stage collar is closed off to prevent a return flow of the liquid slurry into the bore of the liner.
In other completion techniques, an inflatable well packer is disposed in a wellbore on a liner where an inflation liquid is utilized to inflate an elastomer element on the packer and where the elastomer element seals off the annulus of the wellbore. In some instances, it is desirable to have a stage valve above the inflatable packer so that cement can be introduced into the upper annulus between the liner and the wellbore above the inflatable packer.
Stage valves require the ability to remain closed during an initial operations and to be opened only at an appropriate time and to be closed securely at the end of an appropriate time. Stage valves typically include sliding sleeves and latches for retaining the sleeves in one position or another. The sleeves and latches can be mechanically activated or hydraulically activated. However, as with all downhole tools, it is always a problem to determine whether or not a stage collar has safely and reliably performed its function in the wellbore. It is also a problem if the stage valve fails to remain closed.
In most types of cementing operations it is also common to leave cement in the liner which has to be drilled out. Thus, if cementing can be accomplished without leaving cement in the liner, there are substantial economic benefits to the operator.